Double stroller rental fees are so expensive that I have seen people rent a wheelchair and push 2 kids around in it.
I have seen 2 kids in a wheelchair together, but they don't look very comfortable. The rental wheelchairs have a sling seat, so the weight of 2 kids slides them both to the center of the sling, right next to each other (the seat is similar to a director's chair and is made of brown or dark blue vinyl - so not the most comfortable material to sit on).
If the kids are sitting all the way back, the seat is too long and the edge of the seat comes somewhere beyond their knees, so they can't bend their knees. If they sit forward so they can bend their knees over the edge of the seat, the foot rests are way too low for the feet to rest on them (they are not removable or able to be adjusted) and the backrest is too far back to lean against. There are armrest, but depending on the size of the child, they will almost always be too high (anywhere from ear height to shoulder height to mid-upper arms height.
I've seen kids trip when getting out or in because the seat is so high for them and the footrests get in the way. Also, children many of the kids who could get in and out of a stroller by themselves will probably need to be lifted in and out of a rental wheelchair because of the height and the footrests being in the way.
The last thing to keep in mind is that the rental wheelchairs weigh 40-50 pounds empty, so even if your kids are not in it, you will be pushing a pretty heavy weight around the park. IMHO, the money you might save would not be worth the disadvantages of a wheelchair.